Dispersion slope compensating optical waveguide fiber

ABSTRACT

Disclosed is a dispersion compensating and dispersion slope compensating single mode optical waveguide fiber. The refractive index profiles of waveguide fibers in accord with the invention are disclosed and described. These index profiles provide a waveguide fiber having negative total dispersion and negative total dispersion slope so that a standard waveguide fiber is compensated over an extended wavelength range. A telecommunications link using the fiber in accord with the invention is also disclosed and described. A standard fiber to compensating fiber length ratio in the range of 1:1 to 3:1 is shown to give optimum link performance with respect to limiting non-linear dispersion effects.

[0001] This is a divisional of U.S. patent application Ser. No.09/822,168 filed on Mar. 30, 2001, the content of which is relied uponand incorporated herein by reference in its entirety, and the benefit ofpriority under 35 U.S.C. § 120 is hereby claimed. This applicationfurther claims priority to and the benefit of U.S. Provisional PatentApplication No. 60/193,080 filed Mar. 30, 2000.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to an optical waveguidefiber, and particularly to an optical waveguide fiber that compensatesdispersion slope in a telecommunications link.

[0004] 2. Technical Background

[0005] Dispersion compensation techniques in telecommunications systemsor links have been used successfully. A technique useful in linksalready installed is one in which total dispersion (also calledchromatic dispersion) is compensated by an appropriately designedwaveguide fiber formed into a module that can be inserted into the linkat an access point such as an end of the link. The compensatingwaveguide fiber can be designed to allow operation in, for example, the1550 nm operating wavelength window of a link originally designed forthe 1310 nm operating window.

[0006] A disadvantage of compensating with a module is that attenuationand nonlinear penalties are added to the link without increasing theuseful link length. Also some of the refractive index profile designsfor dispersion compensation are more difficult to manufacture and havehigher attenuation than the fibers making up the link.

[0007] Another dispersion compensation scheme is to include bothpositive and negative dispersion fibers in the cables of the link. Eachcable can contain both positive and negative total dispersion waveguidefibers, or the link can be formed using cables having only positivedispersion together with cables having only negative dispersion. Therelatively high attenuation and low effective area of the negativedispersion fiber can be a problem in this scheme as it is in thedispersion compensating module solution. Also the cable inventory mustbe managed carefully, because replacing or repairing a cable involvestracking of another variable (the sign of the dispersion of fibers inthe cable). In certain profile designs a mismatch of mode fields betweenthe positive and negative total dispersion fibers exists and results inexcessive splice or connecting losses.

[0008] There is therefore a need for a total dispersion compensatingstrategy in which the compensating fiber is a part of the link lengthand the problem of the compensating fiber producing excess linkattenuation is addressed. Furthermore, a solution that includesintroducing negative dispersion cabled fiber into the link must offer abenefit that offsets the cost of cable inventory management and thatdoes not introduce excess splice loss into the link.

[0009] A further desired characteristic of a total dispersioncompensation solution is that the compensation be effective over anextended bandwidth to facilitate use of wavelength division multiplexedlink architectures.

Definitions

[0010] The following definitions are in accord with common usage in theart.

[0011] The refractive index profile is the relationship betweenrefractive index or relative refractive index and waveguide fiberradius.

[0012] A segmented core is one that is divided into at least a first anda second waveguide fiber core portion or segment. Each portion orsegment is located along a particular radial length, is substantiallysymmetric about the waveguide fiber centerline, and has an associatedrefractive index profile.

[0013] The radii of the segments of the core are defined in terms of therespective refractive indexes at respective beginning and end points ofthe segments. The definitions of the radii used herein are set forth inthe figures and the discussion thereof.

[0014] Total dispersion, sometimes called chromatic dispersion, of awaveguide fiber is the sum of the material dispersion, the waveguidedispersion, and the inter-modal dispersion. In the case of single modewaveguide fibers the inter-modal dispersion is zero.

[0015] The sign convention generally applied to the total dispersion isas follows. Total dispersion is said to be positive if shorterwavelength signals travel faster than longer wavelength signals in thewaveguide. Conversely, in a negative total dispersion waveguide, signalsof longer wavelength travel faster.

[0016] The effective area is

A _(eff)=2π(∫E ² r dr)²/(∫E ⁴ r dr),

[0017]  where the integration limits are 0 to ∞, and E is the electricfield associated with light propagated in the waveguide.

[0018] The relative refractive index percent, Δ%=100×(n_(i) ²−n_(c)²)/2n_(i) ², where n_(i) is the maximum refractive index in region i,unless otherwise specified, and n_(c) is the average refractive index ofthe cladding region. In those cases in which the refractive index of asegment is less than the average refractive index of the claddingregion, the relative index percent is negative and is calculated at thepoint at which the relative index in most negative unless otherwisespecified.

[0019] The term α-profile refers to a refractive index profile,expressed in terms of Δ (b)%, where b is radius, which follows theequation,

Δ(b)%=Δ(b _(o))(1−[¦b−b _(o)¦/(b ₁ −b _(o))]^(α)),

[0020]  where b_(o) is the point at which Δ(b)% is maximum, b₁ is thepoint at which Δ(b)% is zero, and b is in the range b_(i)≦b≦b_(f), wheredelta is defined above, b_(i) is the initial point of the α-profile,b_(f) is the final point of the α-profile, and α is an exponent which isa real number.

[0021] The pin array bend test is used to compare relative resistance ofwaveguide fibers to bending. To perform this test, attenuation loss ismeasured for a waveguide fiber with essentially no induced bending loss.The waveguide fiber is then woven about the pin array and attenuationagain measured. The loss induced by bending is the difference betweenthe two attenuation measurements. The pin array is a set of tencylindrical pins arranged in a single row and held in a fixed verticalposition on a flat surface. The pin spacing is 5 mm, center to center.The pin diameter is 0.67 mm. The waveguide fiber is caused to pass onopposite sides of adjacent pins. During testing, the waveguide fiber isplaced under a tension just sufficient to make the waveguide conform toa portion of the periphery of the pins. The test pertains to macro-bendresistance of the waveguide fiber.

[0022] A waveguide fiber telecommunications link, or simply a link, ismade up of a transmitter of light signals, a receiver of light signals,and a length of waveguide fiber having respective ends optically coupledto the transmitter and receiver to propagate light signals therebetween.A link can include additional optical components such as opticalamplifiers, optical attenuators, optical switches, optical filters, ormultiplexing or demultiplexing devices. One may denote a group ofinter-connected links as a telecommunications system.

SUMMARY OF THE INVENTION

[0023] One aspect of the present invention is a single mode opticalwaveguide fiber, having a core region and a surrounding clad layer. Thereference to single mode waveguide fiber means that the fiber in cableform usually will carry only a single mode over the range of operatingwavelengths. Persons skilled in the art understand that single modeoperation also includes cases in which more than one mode is propagatedbut that the higher order modes may are strongly attenuated and so donot travel in the waveguide more than a few kilometers. The waveguidefiber in accord with the invention may also be used in a wavelengthrange where a few modes are propagated the full link length and the fewmodes are dispersion compensated. The core region includes at leastthree segments, a central segment beginning at the centerline of thewaveguide fiber, and two annular segments surrounding the centralsegment. In one embodiment, the profile has four segments, a centralsegment, surrounded by a first, second and third annular segment. Eachof the segments is characterized by a refractive index profile, arelative refractive index, and an inner and an outer radius. Therespective segment characteristics are selected to provide a waveguidefiber having a total dispersion at 1550 nm in the range of −30 ps/nm-kmto −60 ps/nm-km and preferably in the range of −30 ps/nm-km to −48ps/nm-km, total dispersion slope at 1550 nm in the range of −0.09ps/nm²-km to −0.18 ps/nm²-km and preferably in the range of −0.09ps/nm²-km to −0.15 ps/nm²-km, an effective area at 1550 nm greater than25 μm², and attenuation at 1550 nm less than or equal to 0.30 dB/km. Ina preferred embodiment, the attenuation at 1550 is less than or equal to0.26 dB/km.

[0024] The respective relative indexes, symbolized beginning at thecentral segment as Δ_(o), the first annular segment (4 in FIG. 1) Δ₁,and second annular segment (6 in FIG. 1) Δ₂, are related by theinequalities, Δ_(o)>Δ₂>Δ₁, and Δ₁<0.

[0025] In an embodiment of the single mode optical waveguide fiber inaccord with the invention, the central segment has relative indexpercent in the range of 0.8% to 1.4% and preferably in the range 0.9% to1.2%, the first annular segment has relative index percent in the rangeof −0.3% to −0.5% and preferably −0.35% to −0.45%, and the secondannular segment has relative index percent in the range of 0.20% to0.45%. The respective radii associated with this embodiment are for thecentral segment an inner radius zero and outer radius, r_(o), in therange 1.8 μm to 3.0 μm, for the first annular segment an inner radiusr_(o) and outer radius in the range r_(o) +1.5 μm to r_(o) +3.0 μm, andfor the second annular segment a center radius in the range 4.5 μm to 10μm and a width, measured between two points defined by the intersectionof the second annular segment refractive index profile with a horizontalline drawn at the half relative index percent value of the secondannular segment refractive index profile, in the range of 0.3 μm to 4.0μm.

[0026] In another embodiment in accord with the invention, the centralsegment of the single mode optical waveguide fiber includes a SiO₂ layerat the interface of the central segment and the first annular segment.This SiO₂ layer is no thicker than 1.5 μm. The composition of the layerranges from pure SiO₂ to 90% SiO₂.

[0027] In a further embodiment of the waveguide fiber profile, there isa flattened region of refractive index beginning at the outer radius ofthe first annular segment. The width of this region is no greater than5.0 μm.

[0028] In yet another embodiment in accord with the invention, the cladlayer adjacent the core region has a refractive index less than that ofSiO₂. This portion of the clad layer has a thickness no greater than 20μm. For most refractive index profile designs of optical waveguidefibers, no light is present at a radius about 20 μm greater than thecore radius.

[0029] A second aspect of the invention is a telecommunications linkwhich makes use of two types of waveguide fibers. A first waveguide typehas positive total dispersion and positive total dispersion slope. Asecond type, made in accord with the invention, has negative totaldispersion and negative total dispersion slope. Combining the two fibertypes in a link allows one to compensate for accumulated positivedispersion in the first waveguide type by using, in the link, anappropriate length of negative total dispersion waveguide fiber. Thedifference in sign of the respective slopes of the first and secondwaveguide types provides for total dispersion compensation over anextended range of operating wavelengths. In addition, the negativedispersion waveguide fiber can provide a net negative dispersion in eachspan which mitigates nonlinear penalties due to modulational instabilityand four wave mixing. This accumulated negative dispersion is thencompensated periodically by single spans of the positive dispersionwaveguide fiber.

[0030] The link includes a transmitter that provides light signals, areceiver that receives the light signals, and at least two lengths ofoptical waveguide fiber optically coupled between the transmitter andreceiver to transport the light signals. At least one of the waveguidefiber lengths has positive total dispersion and total dispersion slope.At least one of the waveguide fiber lengths has negative totaldispersion and negative total dispersion slope. The length, totaldispersion, and total dispersion slope of the fibers are chosen toprovide a link length having a magnitude (as used herein, magnituderefers to absolute value of either a positive or negative totaldispersion or total dispersion slope) of total dispersion and totaldispersion slope less than 10 ps/nm-km and 0.01 ps/nm²-km, respectively.The combination of fibers having total dispersion of different signserve to reduce or eliminate the signal dispersion. Because the fibersalso have total dispersion slope of different sign, the canceling ofsignal dispersion takes place over an extended wavelength range.

[0031] In an embodiment of the link, the signal dispersion cancellationis effective over a wavelength range 1280 nm to 1650 nm so that theoperating window includes wavelengths near 1310 nm as well as the C-band(1530 nm to 1565 nm) and L-band (1565 nm to 1650 nm). The dispersiondata show that operation over this very wide wavelength band ispossible.

[0032] In another embodiment of the link, the optical waveguide fiberhaving positive total dispersion and total dispersion slope is longerthan the optical waveguide fiber having negative total dispersion andtotal dispersion slope. A preferred embodiment is one in which thepositive total dispersion fiber is at least twice as long as thenegative total dispersion fiber. Because of the characteristics of therefractive index profile of the negative total dispersion fiber, thisfiber generally exhibits a higher attenuation relative to that of thepositive total dispersion fiber. Therefore, the link attenuation isreduced when dispersion compensation can be achieved using a shorterlength of negative total dispersion fiber.

[0033] In a further embodiment of the invention, the link is constructedso that the negative total dispersion fiber is farthest from thetransmitter. The advantage of this construction is due to the highereffective area of the positive total dispersion waveguide compared tothe effective area of the negative total dispersion waveguide.Non-linear dispersion effects, such as cross phase modulation and fourwave mixing, are known to depend upon the ratio of power density in thewaveguide fiber to effective to effective area of the fiber. By placingthe higher effective area waveguide fiber nearest the transmitter, thehigher power signal propagates in the larger effective area fiber. Thesignal is attenuated before reaching the lower effective area, negativetotal dispersion fiber so that the non-linear dispersion effects arekept to a minimum.

[0034] In telecommunications links designed for two way signalpropagation in the waveguide fiber, the non-linear effects are minimizedby placing the lower effective area waveguide fiber in the center of thelink between two segments of the link constructed of the highereffective area waveguide fiber.

[0035] Additional features and advantages of the invention will be setforth in the detailed description which follows, and in part will bereadily apparent to those skilled in the art from that description orrecognized by practicing the invention as described herein, includingthe detailed description which follows, the claims, as well as theappended drawings.

[0036] It is to be understood that both the foregoing generaldescription and the following detailed description are merely exemplaryof the invention, and are intended to provide an overview or frameworkfor understanding the nature and character of the invention as it isclaimed. The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate various embodimentsof the invention, and together with the description serve to explain theprinciples and operation of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037]FIG. 1 is an illustration of a refractive index profile thatexhibits the main features of the invention.

[0038]FIG. 2 is an illustration of a refractive index profile of anembodiment of the invention.

[0039]FIG. 3 is an illustration of a refractive index profile of anembodiment of the invention.

[0040]FIG. 4 is a chart showing the comparison between a targetrefractive index profile and a measured refractive index profile of afiber manufactured in accord with the invention.

[0041]FIG. 5 is a chart of total dispersion at 1550 nm versus totaldispersion slope at 1550 nm and total dispersion at 1550 nm versusattenuation at 1550 nm for waveguide fibers made in accord with theinvention.

[0042]FIG. 6 is a chart showing total dispersion versus operatingwavelength for a waveguide fiber manufactured in accord with theinvention.

[0043]FIG. 7 is a chart showing total dispersion slope versus operatingwavelength for a waveguide fiber manufactured in accord with theinvention.

[0044]FIG. 8 is an illustration of a compensated link total dispersionversus wavelength in which the link includes waveguide fibers havingcharacteristics in accord with the invention incorporated at a 2:1 ratioof positive total dispersion fiber to negative total dispersion fiber.

[0045]FIG. 9 is a chart showing total dispersion versus operatingwavelength for a waveguide fiber to be compensated by a waveguide fibermanufactured in accord with the invention.

[0046]FIG. 10 is a chart showing total dispersion slope versus operatingwavelength for a waveguide fiber to be compensated by a waveguidemanufactured in accord with the invention.

[0047]FIG. 11 is an illustration of the relative performance ofwaveguide fiber designed to compensate link total dispersion.

[0048]FIG. 12 is an illustration of a refractive index profile of anembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0049] Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. An exemplary embodiment of the waveguide fiberrefractive index profile of the present invention is shown in FIG. 1.The refractive index profile includes a core region, which begins at thewaveguide fiber centerline and ends at point 18 where the final segmentof the core abuts the surrounding clad layer 20. Central segment 2 has apositive relative index percent, Δ_(o)%, an inner radius of 0 and anouter radius 12 measured from the centerline to the point at which thedescending portion of segment 2 crosses the horizontal axis, i.e., Δ%equal to zero. First annular segment 4 has a negative relative indexΔ₁%, inner radius 12 and outer radius 14 measured from the centerline tothe point at which the ascending portion of segment 4 crosses thehorizontal axis. Second annular segment 6 includes a flattened regionhaving inner radius 14 and width 10, measure from inner radius 14 to thepoint at which the ascending portion of segment 6 begins, and a regionof raised index having center radius 16 and width 8. The center radius16 is measured from the centerline to the point at which the raisedindex is a maximum, that is, at the relative index point Δ₂%. For thoseprofiles having a flattened segment 6, the center radius 16, such asthat shown in FIG. 12, the radius 16 is measured from the centerline tothe center of width 8. The width 8 is measured between the points ofintersection of the half maximum relative index line and the ascendingand descending portions of the raised index region.

[0050] These definitions of radius described in terms of FIG. 1 areapplied consistently to FIGS. 1-4 and 12 which are all illustrations ofrefractive index profiles in accord with the invention. In the interestof brevity and clarity the corresponding parts of the profiles in eachof FIGS. 1-4 and 12 will have the same number and the definitions of theradii and widths will not be repeated.

[0051] The shape of the central segment in FIGS. 1-4 is described asbeing a rounded trapezoid. More particularly, the term rounded trapezoidrefers to a central segment having an inner portion beginning at or nearthe centerline and having a first slope and an adjacent second portionhaving a second slope. The slope of the second portion is greater thanthat of the first portion. The slope of the first portion is in therange 0 to −0.2/μm. In FIG. 12, the shape of the central segment isrounded and can be described as an α-profile having α in the range of 1to 3.

EXAMPLE 1

[0052] A refractive index profile was modeled using the Δ% values andradius values shown in FIG. 1. Central segment 2 has Δ_(o)% of 1.05%,outer radius 1.8 μm, and a rounded trapezoidal shape; first annularsegment 4 has Δ₁% of −0.4%, outer radius 4.0 μm, and a rounded stepshape; second annular segment 6 has a flattened region of width 0.9 μmand relative index near zero (the slight rise in the index of theflattened region is due to dopant diffusion and is accounted for in themodel calculations), and a raised index region Δ₂% of 0.3%, centerradius, 5.6 μm, width 0.85 μm, and a symmetrical rounded shape. Thisshape can be generated using the proper α in the α-profile equation setforth above.

[0053] The modeled properties of this profile are effective area 26 μm²,total dispersion slope −0.11 ps/nm²-km at 1550 nm, total dispersion −39ps/nm-km at 1550 nm, attenuation 0.233 dB/km at 1550 nm, fiber cut offwavelength 1426 nm, and pin array bend loss 4.1 dB.

[0054] The modeled waveguide fiber has respective total dispersion slopeand total dispersion of about 2× that of standard step index single modeoptical waveguide fiber, having a dispersion zero in the 1310 nmoperating window, but of opposite sign. Using a length ratio ofapproximately 2:1, the waveguide fiber of this example can be used tocompensate the total dispersion slope of standard single mode opticalwaveguide fiber, while yielding a residual negative total dispersion.The effective area of the compensating fiber is reasonable and theattenuation and pin array loss is excellent.

COMPARATIVE EXAMPLE 2

[0055] A second refractive index profile was modeled using the Δ% valuesand radius values shown in FIG. 2. Central segment 2 is identical tothat of FIG. 1, annular segment 4 has Δ₁% of −0.42%, outer radius 4.6μm, and a trapezoidal shape having a linear portion of positive slopethat begins at Δ₁% and ends at a relative index of −0.3%; second annularsegment 6 has a flattened region, 10, of width 0.33 μm and relativeindex near zero, and a raised index region Δ₂% of 0.4%, center radius,5.8 μm, width 1.0 μm, and a symmetrical rounded shape. This shape can begenerated using the proper α in the α-profile equation set forth above.

[0056] The modeled properties of this profile are effective area 25 μm²,total dispersion slope −0.16 ps/nm²-km at 1550 nm, total dispersion −36ps/nm-km at 1550 nm, attenuation 0.234 dB/km at 1550 nm, fiber cut offwavelength 1545 nm, and pin array bend loss 3.1 dB.

[0057] These properties are within the desired ranges for a compensatingfiber. It will be understood that the agreement between the modeled andmeasured dispersion properties depends upon the accuracy of the model.Broadening the first annular region and raising the relative index ofthe second annular region serves to improve bend resistance, increasethe total dispersion slope, and increase cut off wavelength. The betterbend resistance is achieved at the cost of a slight decrease ineffective area.

EXAMPLE 3

[0058] Waveguide fiber in accord with the invention was manufactured andhad a refractive index profile as shown in FIG. 4. The target profile isshown as solid line 26 in FIG. 4. The profile as manufactured is shownthe dashed line 28. The close tracking between the target profile andthe manufactured profile shows good process control. Taking the relativeindex percent values and radius values of the dashed line 28, thecentral segment 2 has Δ₀% of 1.05%, outside radius 12 of 2 μm, annularsegment 4 has Δ₁% of −0.42%, outer radius 4.6 μm, and a trapezoidalshape having a linear portion of positive slope that begins at Δ₁% andends at a relative index of −0.33%; second annular segment 6 has aflattened region, 10, of width 0.3 μm and relative index near zero, anda raised index region Δ₂% of 0.4%, center radius 16, 5.3 μm, width 1.0μm, and a symmetrical rounded shape.

[0059] The properties of the waveguide fiber were effective area 26 μm²,total dispersion at 1550 nm, −40 ps/nm-km, total dispersion slope at1550 nm of −0.11 ps/nm²-km, and attenuation at 1550 nm of 0.255 dB/km,in good agreement with the model.

[0060]FIG. 3 shows variations on the embodiment of FIGS. 1 and 2. Alayer of SiO₂ glass 22 can be interposed between the central segment 2and the first annular region 4. The index of refraction of this layercan be slightly above that of SiO₂ due to dopant diffusion from adjacentsegments during manufacturing. The width of the layer is no greater than1.5 μm. In addition, a portion of the clad layer 20 adjacent the secondannular region 6, may be designed to have a refractive index less thanthat of SiO₂, as shown by dashed line 24. The thickness of clad portion24 is less than 20 μm. Layers 22 and 24 provide two additionalparameters to adjust in achieving the desired waveguide fiberproperties. Further, the presence of a clad portion 24 serves toincrease bend resistance. The buffer layer 22 serves to decreasediffusion of index increasing material from central segment 2 into firstannular segment 4.

EXAMPLE 4

[0061] An additional refractive index profile was modeled using the Δ%values and radius values shown in FIG. 12. Central segment 2 can bedescribed by the equation of an α-profile where α is about 1.47. Therelative index Δ_(o)% is 1.08% and the central segment radius is 2.86μm. The profile can be made to exhibit the desired for a range of αvalues. For example a range of α values from 1 to 3 can be used. Annularsegment 4 has Δ₁% of −0.353%, outer radius 4.9 μm, and an α-profileshape; second annular segment 6 has a flattened region, 10, of width 2.5μm and relative index near zero, and a raised, flattened index regionhaving Δ₂% of 0.26%, center radius, 8.95 μm, width 2.9 μm.

[0062] The modeled properties of this profile are effective area 34.1μm², total dispersion slope −0.115 ps/nm²-km at 1550 nm, totaldispersion −32 ps/nm-km at 1550 nm, attenuation 0.215 dB/km at 1550 nm,fiber cut off wavelength 2070 nm, and pin array bend loss 6.58 dB.

[0063] A number of waveguide fibers were made in accord with theinvention and their attenuation, total dispersion, and total dispersionslope measured at 1550 nm. Results of the measurements are shown in FIG.5. The total dispersion ranged from about −34 ps/nm-km to −47 ps/nm-km.Over this total dispersion range, the total dispersion slope remainedessentially constant at −0.10 ps/nm²-km, as can be seen from points 30in FIG. 5. The attenuation at 1550 remained within a range of about 0.24dB/km to 0.33 dB/km over this total dispersion range as shown by points32 in FIG. 5. The data shows the refractive index profile in accord withthe invention to be relatively easily and reproducibly manufactured.

[0064] A telecommunications link was modeled over a wavelength range of1500 nm to 1600 nm using measured properties of both the positive andcompensating negative total dispersion single mode optical waveguidefibers. Curve 40 of FIG. 6 shows the total dispersion of thecompensating fiber as varying between −36 ps/nm-km and −46 ps/nm-km overthe 1500 nm to 1600 nm range. Curve 42 of FIG. 7 shows the totaldispersion slope of the compensating fiber remains within a range −0.09ps/nm²-km to −0.11 ps/nm²-km over this wavelength range.

[0065] Properties of the fiber to be compensated, a standard single modeoptical waveguide fiber as described above, are shown in FIGS. 9 and 10.Curve 44 of FIG. 9 shows the total dispersion of the standard fiber overthe specified wavelength range varies linearly from 16 ps/nm-km at 1500nm to 22 ps/nm-km at 1600 nm. Curve 46 of FIG. 10 shows the totaldispersion slope of the standard fiber over the specified wavelengthrange varies linearly from 0.063 ps/nm²-km at 1500 nm to 0.054 ps/nm²-kmat 1600 nm. A comparison of curve 40 of FIG. 6 to curve 44 of FIG. 9shows the compensation fiber has total dispersion which mirrors thetotal dispersion of the positive dispersion fiber over the wavelengthrange. The total dispersion magnitude of the compensating fiber is abouttwice that of the standard fiber over the wavelength range. A comparisonof curve 42 of FIG. 7 to curve 46 of FIG. 9 shows an analogousrelationship between the respective slopes over the wavelength range ofthe compensating waveguide fiber and the standard waveguide fiber.

[0066] A 44 km system was modeled using the standard and dispersioncompensating waveguide fiber in a length ratio of 2 to 1. The modelingresults are shown in FIG. 8. Curve 34 in FIG. 8 shows the dispersionversus wavelength of the standard fiber. Curve 36 in FIG. 8 shows thedispersion versus wavelength of the compensating fiber. The dispersionversus wavelength of the 44 km link is seen in curve 38 of FIG. 8 to beessentially constant about −2 ps/nm-km. Essentially constant meansdispersion slope magnitude of the link is less than or equal to 0.01ps/nm²-km. The fiber made in accord with the invention provides exactcompensation of the standard fiber over the entire wavelength range 1500nm to 1600 nm.

[0067] Further modeling of waveguide fibers made in accord with theinvention was carried out to determine the relative benefits ofincreasing or decreasing the compensation ratio, that is, the ratio ofthe length of standard fiber to the length of compensating fiber in thelink. Compensating waveguide fibers were designed for use in a modeledlink at a 1:1, 2:1, and 3:1 length ratio. Model results are shown inFIG. 11. The horizontal axis is the compensation ratio, the number 1meaning a length ratio of 1:1, 2 a length ratio of 2:1, and 3 a lengthratio of 3:1. The vertical axis shows normalized power, which is definedas light power P multiplied by the non-linear refractive indexcoefficient n₂ and divided by the effective area A_(eff). That is,normalized power=Pn₂/A_(eff). For three choices of standard fiberattenuation, the 2:1 ratio provided for the lowest normalized power.Curve 48 of FIG. 11, corresponding to a standard fiber attenuation of0.20 dB/km, has a minimum of about 0.135 normalized units at the 2:1compensation point. Curve 49 of FIG. 11, corresponding to a standardfiber attenuation of 0.185 dB/km, has a minimum of about 0.13 normalizedunits at the 2:1 compensation point. Curve 50 of FIG. 11, correspondingto a standard fiber attenuation of 0.175 dB/km, has a minimum of about0.115 normalized units at the 2:1 compensation point. This indicatesthat the non-linear dispersion effects such as modulation instability,four wave mixing, and cross phase modulation will be lowest forcompensating fibers exhibiting a total dispersion and total dispersionslope that results in a 2:1 length ratio in the link, as is the case forthe compensating fibers of the invention. The optimum length ratio liesin the range of 1:1 to 3:1.

[0068] It will be apparent to those skilled in the art that variousmodifications and variations of the present invention can be madewithout departing from the spirit and scope of the invention. Thus, itis intended that the present invention include the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

We claim:
 1. A single mode optical waveguide fiber comprising: a coreregion surrounded by and in contact with a clad layer, wherein the coreregion and the clad layer each have respective refractive index profilesand are configured to guide light through the waveguide fiber; wherein,said core region includes at least three segments, a central segment, afirst annular segment surrounding the central segment, and a secondannular segment surrounding the first annular segment, wherein, eachsaid segment has a respective index profile, inner and outer radius, andrelative index selected to provide a waveguide fiber having a totaldispersion at 1550 nm in the range of −30 ps/nm-km to −60 ps/nm-km,total dispersion slope at 1550 nm in the range of −0.09 ps/nm²-km to−0.18 ps/nm²-km, and attenuation at 1550 nm less than or equal to 0.30dB/km.
 2. The single mode waveguide fiber of claim 1 where the relativeindex of said central segment, Δ_(o), is positive, the relative index ofsaid first annular segment, Δ₁, is negative, the relative index of saidsecond annular segment, Δ₂, is positive, and Δ₀>Δ₂.
 3. The single modewaveguide fiber of claim 1 wherein the attenuation at 1550 nm is lessthan 0.26 dB/km.
 4. The single mode waveguide fiber of claim 1 wherein:said central segment has relative index percent in the range of 0.8% to1.4%; said first annular segment has relative index percent in the rangeof −0.3% to −0.5%; and, said second annular segment has relative indexpercent in the range of 0.20% to 0.45%.
 5. The single mode waveguidefiber of claim 4 wherein: said central segment has inner radius zero andouter radius, r_(o), in the range 1.8 μm to 3.0 μm; said first annularsegment has inner radius r_(o) and outer radius in the range r_(o)+1.5μm to r_(o)+3.0 μm; and said second annular segment has a center radiusin the range 4.5 μm to 10 μm and a width, measured between two pointsdefined by the intersection of the second annular segment refractiveindex profile with a horizontal line drawn at the half relative indexpercent value of the second annular segment refractive index profile, inthe range of 0.3 μm to 4.0 μm.
 6. The single mode waveguide fiber ofclaim 1 or 5 wherein said central segment has a centerline and includesa first portion, beginning at the centerline, having a refractive indexhigher than that of SiO₂, and a second portion located between the firstportion and said first annular region comprising SiO₂, wherein theweight percent SiO₂ of the second portion is greater than or equal to90%.
 7. The single mode waveguide fiber of claim 6 wherein the secondportion has a thickness not greater than 1.5 μm.
 8. The single modewaveguide fiber of claim 1 or 5 wherein said second annular segmentincludes a flattened portion beginning at the outer radius of said firstannular segment and extending outward to a point at which the relativeindex of said second annular portion begins to increase.
 9. The singlemode waveguide fiber of claim 8 wherein the flattened portion has athickness in the range of 0 to 5.0 μm.
 10. The single mode opticalwaveguide fiber of claim 9 wherein the flattened portion is surroundedby a rounded step index portion.
 11. The single mode waveguide fiber ofclaim 1 or 5 wherein the portion of said clad layer adjacent said secondannular region has a refractive index less than that of SiO₂.
 12. Thesingle mode waveguide fiber of claim 11 wherein the portion of said cladlayer having a refractive index less than that of SiO₂ has a thicknessno greater than 20 μm.
 13. A telecommunications link comprising: atransmitter for generating light signals; a receiver for receiving thelight signals; at least two lengths of optical waveguide fiber opticallyjoined in series to form a link length, the link length having a firstend optically coupled to said transmitter and a second end opticallycoupled to said receiver to transport the light signals therebetween;wherein, a first one of said at least two lengths of optical fiber has apositive total dispersion and a positive total dispersion slope, and asecond one of said at least two lengths has a negative total dispersionand negative total dispersion slope, wherein, the first and secondlengths are selected such that said link has a total dispersion ofmagnitude less than 10 ps/nm-km, and a total dispersion slope ofmagnitude less than 0.01 ps/nm²-km, over a pre-selected wavelengthrange.
 14. The telecommunications link of claim 13 wherein thepre-selected wavelength range is 1280 nm to 1650 nm.
 15. Thetelecommunications link of claim 13 wherein the pre-selected wavelengthrange is 1500 nm to 1650 nm.
 16. The telecommunications link of claim 13wherein the first one of the at least two lengths is longer than thesecond one of at least two lengths.
 17. The telecommunications link ofclaim 16 wherein the first one of the at least two lengths and thesecond one of the at least two lengths are present in the link in aratio in the range from 1:1 to 3:1.
 18. The telecommunications link ofclaim 13 wherein at 1550 nm the total dispersion of the first one of theat least two lengths is in the range of 8 ps/nm-km to 25 ps/nm-km andthe total dispersion slope is in the range of 0.05 ps/nm²-km to 0.085ps/nm²-km.
 19. The telecommunications link of claim 18 wherein at 1550nm the total dispersion of the second length of the at least two lengthsis in the range of −30 ps/nm-km to −60 ps/nm-km and the total dispersionslope is in the range of −0.09 ps/nm²-km to −0.18 ps/nm²-km.
 20. Thetelecommunications link of claim 19 wherein the link length is not lessthan 40 km, the pre-selected wavelength range is 1500 nm to 1650 nm, thetotal dispersion magnitude over the range 1500 nm to 1650 nm is lessthan 4 ps/nm-km, and the total dispersion slope magnitude over thewavelength range 1500 nm to 1650 nm is less than 0.01 ps/nm²-km.
 21. Thetelecommunications link of claim 20 wherein the total dispersion slopemagnitude of the link over the range 1500 nm to 1650 nm is less than0.005 ps/nm²-km.
 22. The telecommunications link of claim 20 wherein thetotal dispersion of said link is negative.
 23. The telecommunicationslink of claim 13 wherein the first one of said at least two lengths isoptically coupled to the transmitter.
 24. The telecommunications link ofclaim 23 further including a third length of waveguide fiber havingpositive total dispersion and positive total dispersion slope, whereinsaid third length is optically coupled to the receiver.